The invention relates generally to the field of power over local area networks, particularly Ethernet networks, and more particularly to a method of minimizing harmful transients while probing for a valid powered device (PD) detection signature.
The growth of local and wide area networks, particularly those based on Ethernet technology, has been an important driver for cabling offices and homes with structured cabling systems having multiple twisted wire pairs. The ubiquitous local area network, and the equipment which operates thereon, has led to a situation where there is often a need to attach a network operated device for which power is to be advantageously supplied by the network over the network wiring. Supplying power over the network wiring has many advantages including, but not limited to; reduced cost of installation; centralized power and power back-up; and centralized security and management.
The IEEE 802.3af-2003 standard, whose contents are incorporated herein by reference, is addressed to powering remote devices over an Ethernet based network. In order to avoid supplying power to data terminal equipment not designed to receive power over Ethernet, a probing mode for detecting a valid PD detection signature is incorporated in the standard. The probing mode electrical requirements allow for a maximum open circuit voltage of 30 Volts. Due to inherent capacitance in the power sourcing equipment, the highest voltage achieved while probing for a valid PD detection signature may, in the absence of a valid detected PD, remain resident for some time across the open terminals.
Structured cables, typically comprising 4 twisted wire pairs terminated in a single plug or connector, are commonly used in Ethernet installations. Unfortunately, when connecting or disconnecting equipment equipped with a connector, not all pins in the connector make simultaneous contact. An asymmetric connection, in which only some connector pins make contact, may create an unintended unbalanced transient discharge path which can damage electrical equipment. In particular, the physical layer controller connected at the switch end or in the equipment being connected or disconnected can experience a high transient voltage caused by an unbalanced discharge through the isolation transformer. Such a high transient voltage may result in burn-out of a physical layer controller or destruction of the equipment being connected or disconnected.
It would therefore be desirable to have a method of probing for a valid PD detection signature while preventing harmful voltage and current transients caused by connection or disconnection of powered devices.